It is known that opto-electronic cameras having high resolution and a large field of view generally comprise a strip of photo-sensitive elements (infrared detector), or sometimes a matrix of such photosensitive elements. The photosensitive detector is disposed in the focal plane of an optical device that achieves the desired resolution.
In cameras of this type, the image of the field of view in the focal plane is large relative to the dimensions of the photosensitive detector, thereby making it impossible to take a picture of the entire field of view at a single instant. A moving device for deflecting a light beam is therefore provided, e.g. constituted by at least one mirror, serving to cause the entire image of the field to pass over the detector in the focal plane. In other words, the photosensitive detector constituted by a strip of point photosensitive elements disposed in a plurality of lines and in a few tens or hundreds of columns constitutes a rectangle whose length extends along a constant azimuth angle, over which a strip of the observed field is caused to pass, which strip moves perpendicularly to the length of the rectangle, i.e. azimuth scanning is performed. The image of a different, adjacent strip, is acquired in the same manner by taking up a different elevation angle.
The overall image is thus obtained by juxtaposing a plurality of such strips.
In order to perform such scanning, the deflection device may be constituted, for example, by a mirror that rotates about a first axis of rotation passing substantially through the center of the mirror and disposed so as to be colinear with the axis of the optical device, with rotation about the first axis of rotation defining azimuth displacement relative to the observed field; the mirror is also capable of rotating about a second axis of rotation also passing through the center of the mirror and perpendicular to the first axis of rotation, with the rotary position of the mirror about the second axis defining an elevation displacement relative to the observed field.
For example, the observed field may be scanned in full by giving successive different values to the angle of inclination of the deflection device about the second axis of rotation, and at each of the different angle values, causing the angle of the deflection device to vary continuously between two predetermined extreme values about the first axis of rotation.
In order to ensure that the resolution and the sensitivity of the image obtained from the detector are optimal, it is necessary to ensure that each photosensitive element belonging to a given line of the detector scans the same elements in the observed field by virtue of the motion of the deflector.
Unfortunately, in large-field cameras of the type described above, rotation of the deflection device causes the image of the detector to rotate relative to the observed field about an axis perpendicular to the plane thereof through an angle equal to the angle of rotation of the deflection device about the first axis.
The greater the field, the greater the angle of rotation.
This rotation therefore causes each point in the field to be focused at a position that is dispersed over a plurality of lines of the detector.
An object of the present invention is to remedy this drawback.